Method and apparatus for coordinating fdx and tdd communications in a communication system

ABSTRACT

The method includes transmitting by a first remote communication unit an upstream symbol with a first structure onto a first communication line at a reference time point trf, wherein the reference time point trf is determined based on a time of reception of a downstream symbol with the first structure tFDX_DS_RX and a first propagation delay over the first communication line tPD 1 , as trf=tFDX_DS_RX−tPD 1 ; transmitting by a second remote communication unit an upstream symbol with a second structure onto the second communication line at tTDD_US_TX=trf−tPD 2  during a time interval assigned for upstream transmission on the second communication line, wherein tPD 2  is a second propagation delay over the second communication line, so that the upstream symbol with the second structure transmitted by the second remote communication unit arrives at the access node at the reference time point trf.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 16/643,785, filedMar. 2, 2020, which is a continuation of and claims priority toPCT/EP2018/074255, filed Sep. 10, 2018 which claims priority to EP17191707.3, filed Sep. 18, 2017, the disclosures of each of which arehere by incorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to communication technology, in particular to amethod of coordinating communications in a communication system.

BACKGROUND

Discrete Multi-Tone (DMT) communication paradigm combined withfull-duplex (FDX) transmission (all carriers are simultaneously used forboth directions of communication) has proven to be particularlysuccessful for achieving record-breaking transmission rates over coppermedium, such as Unshielded Twisted Pairs (UTP) or TV broadcast coaxialcables.

FDX transmission can theoretically double the aggregate data ratecompared to Time Division Duplexing (TDD), such as in use for G.fast, orFrequency Division Duplexing (FDD), such as in use for VDSL2.

In FDX, downstream (DS) and upstream (US) symbols are time alignedthough a timing advance (TA) parameter. The aim is to synchronize DS andUS in such a way that both ends of the line start a symbol at the sameabsolute time instance. In this way, the effects of the propagationdelay (PD) are equalized between DS and US, and the cyclic extension(CE) can be minimized to be proportional to 1×PD+delay spread.Conceptually, the CE is split up in a cyclic prefix (CP) to accommodatedelay spread, and a cyclic suffix (CS) to accommodate propagation delay.

In TDD, such as in G.fast standard, DS and US use distinct time slots.The full CE can be used to mitigate delay spread only (CE proportionalto delay spread). The effect of propagation delay only demonstratesitself in requirements on the gap size between US and DS sub-frames.

For a coexistence of FDX and TDD in a communication system, the reach ofthe G.fast line would be severely impacted because part of the cyclicextension of the G.fast TDD line would be used to accommodate for thepropagation delay in order to synchronize the NEXT (near end crosstalk)from the FDX lines and the desired signal from the TDD line.

Thus, an objective of the invention is to propose a synchronizationscheme for a coexistence of FDX and TDD in a communication systemwithout impacting the reach of the G.fast lines.

SUMMARY OF THE INVENTION

The object of the invention is achieved by the method and apparatus inthe claims.

According to one aspect of the present invention, there is provided amethod of coordinating communications in a communication system, thecommunication system comprising an access node, communicatively coupledto: a first set of remote communication units being configured tooperate in a full duplex, FDX, mode via respective ones of a first setof communication lines, and a second set of remote communication unitsbeing configured to operate in a Time Division Duplex, TDD, mode viarespective ones of a second set of communication lines; wherein symbolstransmitted on a first communication line connecting a first remotecommunication unit belonging to the first set of remote communicationunits to the access node have a first structure which comprises a firstcyclic prefix portion, a data portion and a first cyclic suffix portion,and wherein a cyclic extension comprising the first cyclic prefixportion and the first cyclic suffix portion has a predeterminedduration; and symbols transmitted on a second communication lineconnecting a second remote communication unit belonging to the secondset of remote communication units to the access node have a secondstructure which comprises a second cyclic prefix portion and a dataportion, the second cyclic prefix portion having the predeterminedduration, and the symbol with the first structure and the symbol withthe second structure having same symbol duration T_(symb); the methodcomprising: transmitting by the first remote communication unit anupstream symbol with the first structure onto the first communicationline at a reference time point t_(rf), wherein the reference time pointt_(rf) is determined based on a time of reception of a downstream symbolwith the first structure t_(FDX_DS_RX) and a first propagation delayover the first communication line t_(PD1), ast_(rf)=t_(FDX_DS_RX)−t_(PD1); transmitting by the second remotecommunication unit an upstream symbol with the second structure onto thesecond communication line at t_(TDD_US_TX)=t_(rf)−t_(PD2) during a timeinterval assigned for upstream transmission on the second communicationline, wherein t_(PD2) is a second propagation delay over the secondcommunication line.

In a preferred embodiment, the method further comprises: determining thefirst propagation delay t_(PD1) and the second propagation delayt_(PD2); sending the first propagation delay t_(PD1) to the first remotecommunication unit.

In a preferred embodiment, a FDX frame comprising a first FDX sub-frameand a second FDX sub-frame, the first FDX sub-frame comprising a firstnumber of symbols with the first structure, the second FDX sub-framecomprising a second number of symbols with the first structure; a TDDframe comprising a downstream sub-frame and an upstream sub-frame, thedownstream sub-frame comprising the first number of symbols with thesecond structure, the upstream sub-frame comprising the second number ofsymbols with the second structure; the upstream sub-frame is transmittedbefore the second FDX sub-frame is transmitted by an amount of timeequal to the second propagation delay t_(PD2).

In a preferred embodiment, the first FDX sub-frame is a downstreampriority sub-frame where precedence is given to downstreamcommunications from the access node to the first set of remotecommunication units, and the second FDX sub-frame is an upstreampriority sub-frame where precedence is given to upstream communicationsfrom the first set of remote communication units to the access node.

In a preferred embodiment, the method further comprises: determining atime gap t_(g2_TDD) applied at the access node separating an end of adownstream sub-frame transmitted onto the second communication line anda beginning of a subsequent upstream sub-frame received from the secondcommunication line as a predetermined length; determining a time gapt_(g1′_FDX) applied at the first remote communication unit and theaccess node separating an end of a first FDX sub-frame received from thefirst communication line and a beginning of a second FDX sub-frametransmitted onto the first communication line ast_(g1′_FDX)=t_(g2_TDD)−t_(PD1); determining a time gap t_(g1′_TDD)applied at the second remote communication unit separating an end of adownstream sub-frame received from the second communication line and abeginning of a subsequent upstream sub-frame transmitted onto the secondcommunication line as t_(g1′_TDD)=t_(g2_TDD)−2t_(PD2); applying the timegap t_(g1′_FDX) at the first remote communication unit and the accessnode, and applying the time gap t_(g1′_TDD) at the second remotecommunication unit.

In a preferred embodiment, the predetermined length is determinedaccording to the symbol duration T_(symb) and a time gap t_(g1_TDD)applied at the access node separating an end of the upstream framereceived from the second communication line and a beginning of asubsequent downstream sub-frame transmitted onto the secondcommunication line as t_(g2_TDD)=T_(symb)−t_(g1_TDD).

In a preferred embodiment, the first communication line and the secondcommunication line are in a same binder.

In a preferred embodiment, the predetermined duration is determined toaccommodate a delay spread and a propagation delay of a communicationline having a longest supported loop length.

In a preferred embodiment, the method further comprises: controlling theaccess node to transmit a symbol with the first structure to at leastone remote communication unit belonging to the first set of remotecommunication units at the reference time point t_(rf).

According to another aspect of the present invention, there is provideda communication controller for coordinating communications in acommunication system, the communication system comprising an accessnode, communicatively coupled to: a first set of remote communicationunits being configured to operate in a full duplex, FDX, mode viarespective ones of a first set of communication lines, and a second setof remote communication units being configured to operate in a TimeDivision Duplex, TDD, mode via respective ones of a second set ofcommunication lines; wherein symbols transmitted on a firstcommunication line connecting a first remote communication unitbelonging to the first set of remote communication units to the accessnode have a first structure which comprises a first cyclic prefixportion, a data portion and a first cyclic suffix portion, and wherein acyclic extension comprising the first cyclic prefix portion and thefirst cyclic suffix portion has a predetermined duration; and symbolstransmitted on a second communication line connecting a second remotecommunication unit belonging to the second set of remote communicationunits to the access node have a second structure which comprises asecond cyclic prefix portion and a data portion, the second cyclicprefix portion having the predetermined duration, and the symbol withthe first structure and the symbol with the second structure having samesymbol duration T_(symb); the communication controller being configured:to control the first remote communication unit to transmit an upstreamsymbol with the first structure onto the first communication line at areference time point t_(rf), wherein the reference time point t_(rf) isdetermined based on a time of reception of a downstream symbol with thefirst structure t_(FDX_DS_RX) and a first propagation delay over thefirst communication line t_(PD1), as t_(rf)=t_(FDX_DS_RX)−t_(PD1); tocontrol the second remote communication unit to transmit an upstreamsymbol with the second structure onto the second communication line att_(TDD_US_TX)=t_(rf)−t_(PD2) during a time interval assigned forupstream transmission on the second communication line, wherein t_(PD2)is a second propagation delay over the second communication line.

According to another aspect of the present invention, there is providedan access node comprising a communication controller according to thepresent invention.

According to another aspect of the present invention, there is provideda communication system comprising an access node, communicativelycoupled to: a first set of remote communication units being configuredto operate in a full duplex, FDX, mode via respective ones of a firstset of communication lines, and a second set of remote communicationunits being configured to operate in a Time Division Duplex, TDD, modevia respective ones of a second set of communication lines; whereinsymbols transmitted on a first communication line connecting a firstremote communication unit belonging to the first set of remotecommunication units to the access node have a first structure andcomprises a first cyclic prefix portion, a data portion and a firstcyclic suffix portion, and wherein a cyclic extension comprising thefirst cyclic prefix portion and the first cyclic suffix portion has apredetermined duration; and symbols transmitted on a secondcommunication line connecting a second remote communication unitbelonging to the second set of remote communication units to the accessnode have a second structure and comprises a second cyclic prefixportion and a data portion, the second cyclic prefix portion having thepredetermined duration, and the symbol with the first structure and thesymbol with the second structure having same symbol duration T_(symb);wherein communications in the communication system are coordinatedaccording to the present invention.

According to another aspect of the present invention, there is provideda customer Premises Equipment, CPE, characterized in that communicationsof the CPE are coordinated according to the present invention.

The solution in the present invention makes it possible to have acoexistence of FDX and TDD in a communication system without impactingthe reach of the TDD lines.

BRIEF DESCRIPTION OF THE FIGURES

The features and advantages of the invention will be more completelyunderstood by appreciating the following detailed description ofpreferred embodiments with reference to the figures, wherein

FIG. 1 depicts a schematic topology of a communication system accordingto an embodiment of the present invention;

FIG. 2 depicts a schematic timing diagram of an embodiment of thepresent invention;

FIG. 3 depicts a schematic timing diagram of another embodiment of thepresent invention.

Wherein same or similar reference numerals refer to same or similarparts or components.

DETAILED DESCRIPTION

Exemplary embodiments of the present application are described herein indetail and shown by way of example in the drawings. It should beunderstood that, although specific exemplary embodiments are discussedherein there is no intent to limit the scope of the invention to suchembodiments. To the contrary, it should be understood that the exemplaryembodiments discussed herein are for illustrative purposes, and thatmodified and alternative embodiments may be implemented withoutdeparting from the scope of the invention as defined in the claims.Similarly, specific structural and functional details disclosed hereinare merely representative for purposes of describing the exemplaryembodiments. The invention described herein, however, may be embodied inmany alternate forms and should not be construed as limited to only theembodiments set forth herein.

FIG. 1 shows a schematic topology of a communication system according toan embodiment of the present invention.

As shown in FIG. 1, the communication system 100 comprises an accessnode 110, a first set of remote communication units 121, 122 and asecond set of remote communication units 131, 132.

Specifically, the access node 110 may be implemented as a DistributionPoint Unit operating according to G.fast legacy TDD and a new FDXcommunication technology to be deployed in co-existence with the legacyTDD G.fast communication technology. The access node 110 is typicallydeployed at a remote location closer to subscriber premises, in a streetcabinet, on a pole, in the basement of a building, etc.

The first set of remote communication units 121, 122 are configured tooperate in the FDX mode. The second set of remote communication units131, 132 are configured to operate in the TDD mode.

In one embodiment, the access node 110 comprises a first set ofcommunication units 111, 113 operating in the FDX mode connected throughrespective ones of a first set of communication lines to the first setof remote communication units 121, 122. The access node 110 furthercomprises a second set of communication units 112, 114 operating in theTDD mode connected through respective ones of a second set ofcommunication lines to the second set of remote communication units 131,132. The communication lines are typically copper Unshielded TwistedPair (UTP).

In one embodiment, the access node 110 further comprises a communicationcontroller 115 for coordinating communications in the communicationsystem 100.

Specifically, in the following, embodiments of the invention will bedescribed with respect to a first communication line and a secondcommunication line. The first communication line connects twocommunication units operating in the FDX mode, i.e., a first remotecommunication unit 121 and a first communication unit 111 in the accessnode 110. The second communication line connects two communication unitsoperating in the TDD mode, i.e., a second remote communication unit 131and a second communication unit 112 in the access node 110.Advantageously, the first communication line and the secondcommunication line are in the same binder.

Specifically, the communication units 111 and 112 at the access node,the first remote communication unit 121 and the second remotecommunication unit 131 individually comprise a transmitter (TX) and areceiver (RX). The first remote communication unit 121 and the secondremote communication unit 131 may be implemented in a Customer PremisesEquipment (CPE). By way of example, the CPE may be implemented as aG.fast gateway, a router, a bridge, or a Network Interface Card (NIC).

FIG. 2 shows a schematic timing diagram of an embodiment of the presentinvention.

As shown in FIG. 2, symbols transmitted on the first communication linein both directions, i.e., upstream direction (from the first remotecommunication unit 121 to the access node 110) and downstream direction(from the access node 110 to the first remote communication unit 121)have a first structure. A symbol with the first structure comprises afirst cyclic prefix portion CP1, a data portion and a first cyclicsuffix portion CS1, and wherein a cyclic extension CE comprising thefirst cyclic prefix portion CP1 and the first cyclic suffix portion CS1has a predetermined duration t_(CE).

Symbols transmitted on the second communication line in both directions,i.e., upstream direction (from the second remote communication unit 131to the access node 110) and downstream direction (from the access node110 to the second remote communication unit 131) have a secondstructure. A symbol with the second structure comprises a second cyclicprefix portion CP2 and a data portion, the second cyclic prefix portionCP2 having the predetermined duration t_(CE).

Symbols with the second structure may further include a small cyclicsuffix portion (not shown) for windowing purpose.

Besides, the symbol with the first structure and the symbol with thesecond structure have the same symbol duration T_(symb). Therefore, theduration of the data portion in the symbol with the first structureequals the duration of the data portion in the symbol with the secondstructure.

In a preferred embodiment, the predetermined duration t_(CE) isdetermined to accommodate a delay spread and a propagation delay of acommunication line having a longest supported loop length. For a symbolwith the first structure, the duration of the cyclic prefix portion CP1is determined to accommodate the delay spread, and the duration of thecyclic suffix portion CS1 is determined to accommodate the propagationdelay.

According to an embodiment of the present invention, the communicationcontroller 115 is configured to control the first remote communicationunit 121 and the access node 110 to transmit a symbol with the firststructure onto the first communication line at the same absolute time,so as to equalize the effect of propagation delay between DS and US andto maximize the supported loop length.

Specifically, the communication controller 115 is configured to controlthe access node 110 to transmit a downstream symbol with the firststructure (FDX_DS) to the first remote communication unit 121 at areference time point t_(rf), namely, t_(FDX_DS_TX)=t_(rf). In theembodiment shown in FIG. 2, t_(rf)=0.

Accordingly, the FDX_DS symbol transmitted by the access node 110arrives at the first remote communication unit 121 att_(FDX_DS_RX)=t_(rf)+t_(PD1), wherein, t_(PD1) is a first propagationdelay over the first communication line.

In one embodiment, the first remote communication unit 121 issynchronized with the access node 110 based on the time of reception ofthe FDX_DS symbol t_(FDX_DS_RX) and the first propagation delay t_(PD1).Therefore, the reference time t_(rf) can be determined the first remotecommunication unit 121 as t_(rf)=t_(FDX_DS_RX)−t_(PD1).

In one embodiment, the first propagation delay is determined by theaccess node 110 and transmitted to the first remote communication unit121. Alternatively, the first propagation delay can be directlydetermined by the first remote communication unit 121.

After the reference time t_(rf) is determined at the first remotecommunication unit 121, the first remote communication unit 121transmits an upstream symbol with the first structure (FDX_US) onto thefirst communication line at the reference time point t_(rf). The FDX_USsymbol transmitted by the first remote communication unit 121 arrives atthe access node 110 at t_(FDX_US_RX)=t_(rf)t_(PD1).

Furthermore, the communication controller 115 is configured to controlthe second remote communication unit 131 to transmit an upstream symbolwith the second structure (TDD_US) onto the second communication line att_(TDD_US_TX)=t_(rf)−t_(PD2) during a time interval assigned forupstream transmission on the second communication line, wherein t_(PD2)is a second propagation delay over the second communication line. As aresult, the TDD_US symbol transmitted by the second remote communicationunit 131 arrives at the access node 110 at the reference time pointt_(rf).

At the access node 110, a RX FFT window has a predetermined durationwhich equals the duration of the data portion of a symbol. As shown inFIG. 2, The RX FFT window begins at a time point when the data portionof the TDD_US symbol begins. As t_(PD1)≤t_(CS1) and as the cyclic suffixportion CS1 also contains a cyclic extension of the data portion, it ispossible to find a RX FFT window that does not include any symboltransition for the direct receive symbol and for the symbols receivedthrough interference (ECHO, NEXT, FEXT), while remaining away from thecyclic prefix portion CP1 and corresponding Inter Symbol Interference(ISI). In this way, mutual orthogonality between carriers is guaranteed.Typically, the RX FFT window is chosen to be as far as possible from thecyclic prefix portion CP1 as depicted in FIG. 2.

According to the present invention, it is possible to combine the FDXsymbol structure comprising the first cyclic prefix portion and firstcyclic suffix portion, which is necessary to realize FDX operation forthe FDX lines, with a maximal cyclic prefix portion for the TDD symbols.This maximal cyclic prefix portion results in the maximal possible reachfor the TDD lines for the given cyclic extension length.

In FIG. 2, the present invention is elaborated with respect to symbols.In operation, a plurality of symbols are transmitted consecutively in asub-frame. In the following, the present invention will be describedwith respect to sub-frames.

FIG. 3 shows a schematic timing diagram of another embodiment of thepresent invention.

As shown in FIG. 3, a TDD frame transmitted on the second line comprisesa downstream sub-frame, an upstream sub-frame and necessary guard timethere between.

The communication controller 115 is configured to determine a time gapt_(g2_TDD) applied at the access node 110 separating an end of adownstream sub-frame transmitted onto the second communication line anda beginning of a subsequent upstream sub-frame received from the secondcommunication line as a predetermined length.

In one embodiment, the predetermined length is determined according tothe symbol duration T_(symb) and a time gap t_(g1_TDD) applied at theaccess node 110 separating an end of the upstream frame received fromthe second communication line and a beginning of a subsequent downstreamsub-frame transmitted onto the second communication line. Specifically,there is t_(g2_TDD)=T_(symb)−t_(g1_TDD).

In FIG. 3, the downstream sub-frame comprises M_(ds) downstream symbolswith the second structure, and the upstream sub-frame comprises M_(us)upstream symbols with the second structure. Thereby, the duration of aTDD frame T_(F_TDD) may be expressed asT_(F_TDD)=(M_(ds)+M_(us)+1)×T_(symb).

Then, according to FIG. 3, a time gap t_(g1′_TDD) applied at the secondremote communication unit 131 separating an end of a downstreamsub-frame received from the second communication line and a beginning ofa subsequent upstream sub-frame transmitted onto the secondcommunication line may be determined as t_(g1′_TDD)=t_(g2_TDD)−2t_(PD2).In one embodiment, the time gap t_(g1′_TDD) may be transmitted to thesecond remote communication unit 131, so that it may be applied at thesecond remote communication unit 131.

In the embodiment shown in FIG. 3, FDX coexists with legacy TDD in acommunication system, and a FDX frame is set to be of same length as thelegacy TDD frame T_(F_FDX)=T_(F_TDD)=(M_(ds)+M_(us)+1)×T_(symb).Specifically, the FDX frame in FIG. 3 comprises a first FDX sub-frame asecond FDX sub-frame and necessary guard time there between. The firstFDX sub-frame comprises M_(ds) symbols with the first structure, and thesecond FDX sub-frame comprises M_(us) symbols with the first structure.Thus, the first FDX sub-frame has the same duration as the downstreamsub-frame in the second line, and the second FDX sub-frame has the sameduration as the upstream sub-frame in the second line.

Specifically, in one embodiment, the first and second FDX sub-frames maycorrespond to unprioritized FDX transmission.

In another embodiment, the first FDX sub-frame is a downstream prioritysub-frame where precedence is given to downstream communications fromthe access node 110 to the first set of remote communication units 121,122, and the second FDX sub-frame is an upstream priority sub-framewhere precedence is given to upstream communications from the first setof remote communication units 121, 122 to the access node 110, forinstance as explained in EP application No 16306744.0 entitled “MethodAnd Apparatus For Full-Duplex Communication over Wired TransmissionMedia” filed by Alcatel-Lucent on Dec. 20, 2016.

In FIG. 3, the vertical dashed lines represent absolute time alignedacross the access node 110, the first remote communication unit 121 andthe second remote communication unit 131.

The vertical dashed line on the left represents the starting time of aTDD frame and a FDX frame. As can be seen from FIG. 1, the TDD frame andthe FDX frame start at the same time. During the duration of adownstream sub-frame, symbols transmitted onto the first communicationline and the second communication line are aligned with respect to theirtime of transmission.

The vertical dashed line in the middle aligns the time of reception ofan upstream sub-frame and the time of transmission of a second FDXsub-frame. As can be seen from FIG. 3, the upstream sub-frame istransmitted before the second FDX sub-frame by an amount of time equalto the second propagation delay t_(PD2). Thus, the upstream sub-frame isreceived at the same time as the second FDX sub-frame is transmitted.

The vertical dashed line on the right represents the starting time of asubsequent TDD frame and FDX frame.

In order to make sure that the second FDX sub-frame is transmitted atthe same time as the upstream sub-frame is received, the communicationcontroller 115 is further configured to determine a time gap t_(g1′_FDX)applied at the first remote communication unit 121 and the access node110 separating an end of a first FDX sub-frame received from the firstcommunication line and a beginning of a subsequent second FDX sub-frametransmitted onto the first communication line ast_(g1′_FDX)=t_(g2_TDD)−t_(PD1).

The communication controller 115 is further configured to apply the timegap t_(g1′_FDX) at the first remote communication unit 121 and theaccess node 110. In one embodiment, the time gap t_(g1′_TDD) istransmitted to the second remote communication unit 131, so that it maybe applied at the second remote communication unit 131.

1. A method of coordinating communications in a communication system,the communication system comprising an access node, communicativelycoupled to a first set of remote communication units being configured tooperate in a full duplex, FDX, mode via respective ones of a first setof communication lines, and a second set of remote communication unitsbeing configured to operate in a Time Division Duplex, TDD, mode viarespective ones of a second set of communication lines; wherein symbolstransmitted on a first communication line connecting a first remotecommunication unit belonging to the first set of remote communicationunits to the access node have a first structure and symbols transmittedon a second communication line connecting a second remote communicationunit belonging to the second set of remote communication units to theaccess node have a second structure and the symbol with the firststructure and the symbol with the second structure having same symbolduration T_(symb); wherein said method of coordination comprises:controlling that an upstream symbol with the second structure isreceived by the access node from the second communication line at a sametime as a downstream symbol with the first structure is transmitted bythe access node onto the first communication line.
 2. A method accordingto claim 1, wherein a FDX frame comprises a first FDX sub-frame and asecond FDX sub-frame, the first FDX sub-frame comprising a first numberof symbols with the first structure, the second FDX sub-frame comprisesa second number of symbols with the first structure; and a TDD framecomprises a downstream sub-frame and an upstream sub-frame, thedownstream sub-frame comprising the first number of symbols with thesecond structure, the upstream sub-frame comprising the second number ofsymbols with the second structure.
 3. Method according to claim 2further comprising: determining a propagation delay t_(PD2) over thesecond communication line, controlling the transmission of an upstreamTDD sub-frame by the second remote communication unit and thetransmission of a second FDX sub-frame by the access node such that theupstream TDD sub-frame is transmitted before the transmission of thesecond FDX sub-frame by an amount of time equal to the propagation delayt_(PD2).
 4. Method according to claim 2 further comprising: determininga first time gap t_(g1′_FDX) to be applied at said first remotecommunicate node, said first time gap separating an end of a first FDXsubframe received at the first communication unit and a beginning of asubsequent second FDX upstream subframe transmitted onto the firstcommunication line by the first remote communication unit.
 5. A methodaccording to claim 4, further comprising: determining a firstpropagation delay t_(PD1) over the first communication line determininga second time gap t_(g2_TDD) to be applied at the access node 110separating an end of a downstream sub-frame transmitted onto the secondcommunication line and a beginning of a subsequent upstream sub-framereceived from the second communication line; determining a secondpropagation delay t_(PD2) over the second communication line,controlling the transmission of an upstream TDD sub-frame by the secondremote communication unit and the transmission of a second FDX sub-frameby the access node such that the upstream TDD sub-frame is transmittedbefore the transmission of the second FDX sub-frame by an amount of timeequal to the second propagation delay t_(PD2); and determining saidfirst time gap t_(g1′_FDX) as t_(g1′_FDX)=t_(g2_TDD)−t_(PD1)
 6. A methodaccording to claim 2, wherein, the first FDX sub-frame is a downstreampriority sub-frame where precedence is given to downstreamcommunications from the access node to the first set of remotecommunication units, and the second FDX sub-frame is an upstreampriority sub-frame where precedence is given to upstream communicationsfrom the first set of remote communication units to the access node. 7.A method according to claim 5, further comprising: determining a thirdtime gap t_(g1′_TDD) to be applied at the second remote communicationunit for separating an end of a downstream sub-frame received from thesecond communication line and a beginning of a subsequent upstreamsub-frame transmitted onto the second communication line ast_(g1′_TDD)=t_(g2_TDD)—2t_(PD2).
 8. A method according to claim 2,wherein the method comprises: determining a first time gap t_(g1_TDD)applied at the access node separating an end of the upstream sub-framereceived from the second communication line and a beginning of asubsequent downstream sub-frame transmitted onto the secondcommunication line.
 9. Method according to claim 8 wherein a second timegap t_(g2_TDD) is determined as t_(g2_TDD)=T_(symb)−t_(g1_TDD), withsaid second time gap t_(g2_TDD) separates an end of a downstreamsub-frame transmitted onto the second communication line and a beginningof a subsequent upstream sub-frame received from the secondcommunication line at the access node.
 10. A method according to claim1, wherein the first communication line and the second communicationline are in a same binder.
 11. A method according to claim 1 furthercomprising: controlling the first remote communication unit and thesecond remote communication unit such that the first remotecommunication unit will transmit an upstream symbol with the firststructure onto the first communication line at a reference time pointt_(rf), wherein the reference time point t_(rf) is determined based on atime of reception t_(FDX_DS_RX) of a downstream symbol with the firststructure and the first propagation delay over the first communicationline t_(PD1), as t_(rf)=t_(FDX_DS_RX)−t_(PD1); and the second remotecommunication unit will transmit an upstream symbol with the secondstructure onto the second communication line att_(TDD_US_TX)=t_(rf)−t_(PD2) during a time interval assigned forupstream transmission on the second communication line, wherein t_(PD2)is the second propagation delay over the second communication line. 12.A method according to claim 11 further comprising: controlling theaccess node to transmit a symbol with the first structure to at leastone remote communication unit belonging to the first set of remotecommunication units at the reference time point t_(rf).
 13. Acommunication controller for coordinating communications in acommunication system, the communication system comprising an accessnode, communicatively coupled to a first set of remote communicationunits being configured to operate in a full duplex, FDX, mode viarespective ones of a first set of communication lines, and a second setof remote communication units being configured to operate in a TimeDivision Duplex, TDD, mode via respective ones of a second set ofcommunication lines; wherein symbols transmitted on a firstcommunication line connecting a first remote communication unitbelonging to the first set of remote communication units to the accessnode have a first structure and symbols transmitted on a secondcommunication line connecting a second remote communication unitbelonging to the second set of remote communication units to the accessnode have a second structure and the symbol with the first structure andthe symbol with the second structure having same symbol durationT_(symb); the communication controller being configured to: control thatan upstream symbol with the second structure is received by the accessnode from the second communication line at the same time as a downstreamsymbol with the first structure is transmitted by the access node ontothe first communication line.
 14. Communication controller of claim 13wherein the communication controller is included in an access node. 15.A customer premises equipment apparatus comprising: at least oneprocessor; and at least one memory including computer program code; theat least one memory and the computer program code configured to, withthe at least one processor, cause the apparatus at least to performoperate in a full duplex, FDX, mode when receiving and/or transmittingsignals over a first set of communication lines or operate in a TimeDivision Duplex, TDD, mode; when receiving or transmitting signals froma second set of communication lines; with symbols of the FDX mode havinga first structure and symbols of the TDD mode having a second structure,receive instructions from a communication controller for controllingoperation of said apparatus, such that, when operating in a full duplexFDX mode, transmitting an upstream symbol with the first structure at areference time point t_(rf), and when operating in a time divisionduplex TDD mode, transmitting an upstream symbol with the secondstructure at t_(TDD_US_TX)=t_(rf)−t_(PD2) during a time intervalassigned for upstream transmission on the second communication line,wherein t_(PD2) is a propagation delay in said second mode and receivedfrom said communication controller.
 16. An access node, comprising: atleast one processor; and at least one memory including computer programcode; the at least one memory and the computer program code configuredto, with the at least one processor, cause the access node at least toperform operate in a full duplex, FDX, mode when receiving and/ortransmitting signals over a first set of communication lines and operatein a Time Division Duplex, TDD, mode when receiving or transmittingsignals from a second set of communication lines, wherein symbolstransmitted on a first communication line of the first set ofcommunication lines have a first structure and symbols transmitted on asecond communication line of the second set of communication lines havea second structure, and symbols with the first structure and symbolswith the second structure have same symbol duration T_(symb); andreceive an upstream symbol with the second structure over acommunication line of said second set at same time of transmitting adownstream symbol with the first structure over a communication line ofthe first set.
 17. An access node according to claim 16 wherein the atleast one memory and the computer program code are configured to, withthe at least one processor, cause the access node at least to furtherperform determining a first propagation delay t_(PD1) over thecommunication line of said first set, determining a second propagationdelay t_(PD2) over the communication line of said second set,determining a time gap t_(g2_TDD) for separating an end of a downstreamsub-frame transmitted onto the communication line of said second set anda beginning of a subsequent upstream sub-frame received from thecommunication line of said second set, such that a FDX frame comprises afirst FDX sub-frame and a second FDX sub-frame, the first FDX sub-framecomprising a first number of symbols with the first structure, thesecond FDX sub-frame comprises a second number of symbols with the firststructure; and a TDD frame comprises a downstream sub-frame and anupstream sub-frame, the downstream sub-frame comprising the first numberof symbols with the second structure, the upstream sub-frame comprisingthe second number of symbols with the second structure.
 18. Access nodeaccording to claim 17 wherein the at least one memory and the computerprogram code are configured to, with the at least one processor, causethe access node at least to further perform determining a another timegap t_(g1_TDD) for separating an end of the upstream sub-frame receivedfrom the second communication line and a beginning of a subsequentdownstream sub-frame transmitted onto the second communication line.